A low temperature is below 0° C., preferably below −40° C.
A mixture rich in carbon dioxide contains at least 60 mol % of carbon dioxide or even at least 80 mol % of carbon dioxide.
The remainder of the mixture may contain one or more of the following components: oxygen, nitrogen, argon, nitrogen oxide (NO, NO2 or N2O), carbon monoxide, hydrogen, mercury.
The purification may be carried out by one or more successive partial condensation steps and/or by distillation.
US-A-2010/0326134 describes a process according to the preamble of claim 1. WO-A-2012/030223 describes a process according to the preamble of claim 1 except that it does not mention the technology used for the first heat exchanger.
In the prior art, the process used for purifying CO2 in a large amount consists in cooling the CO2-rich gas mixture to the temperature as close as possible to the freezing of the CO2 (−56.5° C.) in order to condense the maximum amount of CO2. The cold is provided in general by the vaporization of the CO2 recovered and the heating of the various fluids, for example the vaporized CO2, the noncondensable gases, or various recycling streams. Advanced thermal integration makes it possible to reduce the energy consumption of the process. Thus, brazed aluminum plate exchangers are particularly well suited for cooling the CO2-rich gas mixture since they permit heat exchanges between numerous fluids (6 is a common number) with a small temperature difference between the hot fluids and the cold fluids (a difference of 2° C. is common practice also).
The coldest temperature of the exchange is provided by the vaporization of CO2 close to the triple point. This vaporization thus has the risk of making a solid CO2 phase appear if the pressure drops to the triple point value. Indeed, below the pressure of the triple point (5.185 bar a), liquid CO2 cannot exist and it is then converted to a solid phase (for around 60% of the molecules) and a gas phase. The vaporization of the liquid provides the cold necessary for solidifying the rest.
This solid phase, which may appear upon a fluctuation of the pressure of the vaporized gas, for example if the compressor which recovers it sucks up more molecules than vaporized in the exchanger, may block the exchanger and damage it during heating phases. This is a limitation for the use of a brazed aluminum exchanger.